This invention relates to cellular grids for positioning nuclear fuel rods, or the like in close proximity to one another. As will be known a nuclear fuel element comprises an assembly of nuclear fuel rods held together in a cluster so that it may be handled as a unit and, in order to ensure correct spacing between adjacent rods over their length, cellular grids are employed. Generally these grids are usually one of two kinds. Those of one kind are constructed from metal strips which run parallel to one another in two directions at right angles so as to form cells between intersecting strips of adjacent rows. Those of a second kind are constituted by a group of co-joined cylindrical or tubular ferrules each ferrule being adapted to receive a nuclear fuel rod and having received it, to hold it in its correct position. The present invention is concerned with grids of the second kind and aims to provide an improved construction of the ferrule type grid.